Known in the prior art is a spiral drill comprising chip flutes set at an acute angle to the drill axis and having, each, a leading surface which, on intersecting the rear drill sharpening surface, forms the major cutting edge, a flank surface merging into the land, and a mating surface of the leading and flank surfaces, contacting the drill core (DA, B, 2144217).
This drill has a large body of tool material, limited by the internal surfaces of helical chip flutes and lands and, by virtue of this fact, the chip flutes have a limited small volume included between the external surfaces of helical chip flutes and the surface of the hole walls. Due to such an arrangement the process of drilling holes in parts of low-strength materials, e.g. foil-coated dielectric (glass-base textilite, paper-based laminate, etc.), plastics, plexiglas, ligth alloys, etc. producing an abundance of chips fails to ensure sufficient disposal of chips.
While handling such materials, the chips stick and clog the chip flutes making it necessary to stop work, clean out of replace the drill; besides, in case of small-diameter holes, particularly smaller 3 mm, the drills may even break. All this, taken together, reduces the drilling efficiency.
The lands of such a drill protrude above the flank surface of helical chip flutes and have a considerable width. This increases friction and heating of both the drill proper and the walls of the hole in the course of drilling. Heating of the drill brings about a sharp deterioration of its strength particularly when drilling holes in the parts made of glass-base textolite, paper-based laminate, plastics and other materials with a low heat conductivity.
The leading surface of the helical flutes in such a drill forms, together with the surface of lands, an angle approaching 90.degree. which hinders the cutting process considerably.
Thus, heating of the hole walls and difficult cutting result in a considerable roughness and poor quality of holes machined in the parts of viscous, low-strength materials, particularly in printed circuit boards made of foil-coated dielectrics containing solidified epoxy resins having a tendency towards burns, melting and spreading over the hole walls. By virtue of the above reasons, the drilled printed circuit boards have such characteristic defects as burrs or exfoliation of foil on the hole edges, tousling of glass fibres or their tear-out from the hole surface. These defects cannot always be corrected by additional machining and reduce the quality of subsequent metallization of holes, particularly in printed circuit boards.
Besides, manufacture of such a drill involves certain technological complications due to the fact that the helical chip flutes have a complex curvilinear shape with a charp elevation at the point where the flank surface of helical flutes merges into a land. This complicates the shape of the disc tool for machining helical flutes.